1. Field of the Invention
The invention is related to the field of instruments used to sample fluids contained in the pore spaces of earth formations. More specifically, the invention is related to methods of determining hydraulic properties of anisotropic earth formations by interpreting fluid pressure and flow rate measurements made by such instruments.
2. Description of the Related Art
Electric wireline formation testing instruments are used to withdraw samples of fluids contained within the pore spaces of earth formations and to make measurements of fluid pressures within the earth formations. Calculations made from these pressure measurements and measurements of the withdrawal rate can be used to assist in estimating the total fluid content within a particular earth formation.
A typical electric wireline formation testing instrument is described, for example, in U.S. Pat. No. 5,377,755 issued to Michaels et al. Electric wireline formation testing instruments are typically lowered into a wellbore penetrating the earth formations at one end of an armored electrical cable. The formation testing instrument usually comprises a tubular probe which is extended from the instrument housing and then is impressed onto the wall of the wellbore. The probe is usually sealed on its outside diameter by an elastomeric seal or packing element to exclude fluids from within the wellbore itself from entering the interior of the probe, when fluids are withdrawn from the earth formation through the probe. The probe is selectively placed in hydraulic communication, by means of various valves, with sampling chambers included in the instrument. Hydraulic lines which connect the probe to the various sample chambers can include connection to a highly accurate pressure sensor to measure the fluid pressure within the hydraulic lines. Other sensors in the instrument can make measurements related to the volume of fluid which has entered some of the sample chambers during a test of a particular earth formation. U.S. Pat. No. 6,478,096 to Jones et al. discloses a formation pressure tester that is part of a bottomhole assembly used in drilling and can make measurements while drilling (MWD).
Properties of the earth formation which can be determined using measurements made by the wireline formation testing instrument include permeability of the formation and static reservoir pressure. Permeability is determined by, among other methods, calculating a rate at which a fluid having a known viscosity moves through the pore spaces within the formation when a predetermined differential pressure is applied to the formation. As previously stated, the formation testing instrument typically includes a sensor to make measurements related to the volume of fluid entering the sample chamber, and further includes a pressure sensor which can be used to determine the fluid pressure in the hydraulic lines connecting the probe to the sample chamber. It is further possible to determine the viscosity of the fluid in the earth formation by laboratory analysis of a sample of the fluid which is recovered from the sample chamber.
The permeability of a reservoir is an important quantity to know as it is one of the important factors determining the rate at which hydrocarbons can be produced from the reservoir. Historically, two types of measurements have been used for determination of permeability. In the so-called drawdown method, a probe on a downhole tool in a borehole is set against the formation. A measured volume of fluid is then withdrawn from the formation through the probe. The test continues with a buildup period during which the pressure is monitored. The pressure measurements may continue until equilibrium pressure is reached (at the reservoir pressure). Analysis of the pressure buildup using knowledge of the volume of withdrawn fluid makes it possible to determine a permeability.
In the so-called buildup method, fluid is withdrawn from the reservoir using a probe and the flow of fluid is terminated. The subsequent buildup in pressure is measured and from analysis of the pressure, a formation permeability is determined.
U.S. Pat. No. 5,708,204 to Kasap having the same assignee as the present application and the contents of which are fully incorporated herein by reference, teaches the Fluid Rate Analysis (FRA) method in which data from a combination of drawdown and buildup measurements are used to determine a formation permeability.
The methods described above give a single value of permeability. In reality, the permeability of earth formations is anisotropic. It is not uncommon for horizontal permeabilities to be ten or more times greater than the vertical permeability. Knowledge of both horizontal and vertical permeabilities is important for at least two reasons. First, the horizontal permeability is a better indicator of the productivity of a reservoir than an average permeability determined by the methods discussed above. Secondly, the vertical permeability provides useful information to the production engineer of possible flow rates between different zones of a reservoir, information that is helpful in the setting of packers and of perforating casing in a well. It is to be noted that the terms “horizontal” and “vertical” as used in the present document generally refers to directions in which the permeability is a maximum and a minimum respectively. These are commonly, but not necessarily horizontal and vertical in an earth reference frame. Similarly, the term “horizontal” in connection with a borehole is one in which the borehole axis is parallel to a plane defined by the horizontal permeability.
U.S. Pat. No. 4,890,487 to Dussan et al. teaches a method for determining the horizontal and vertical permeabilities of a formation using measurements made with a single probe. The analysis is based on representing the fluid behavior during drawdown by an equation of the form:
                                                        P              f                        -                          P              i                                =                      (                                                            Q                  ⁢                                                                          ⁢                  μ                                                  2                  ⁢                  π                  ⁢                                                                          ⁢                                      r                    p                                    ⁢                                      k                    h                                                              ⁢                              F                ⁡                                  (                                                            π                      2                                        ,                                                                  1                        -                                                                              k                            V                                                    /                                                      k                            H                                                                                                                                )                                                      )                          ,                            (        1        )            where    Pf represents pressure of the undisturbed formation;    Pi represents pressure at the end of draw-down period i;    Qi represents volumetric flow rate during draw-down period i;    μ represents dynamic viscosity of the formation fluid;    rp represents the probe aperture radius;    kH represents horizontal formation permeability;    kV represents vertical formation permeability; and    F denotes the complete elliptic integral of the first kind.In Dussan, at least three sets of measurements are made, such as two drawdown measurements and one buildup measurement, and results from these are combined with a table lookup to give an estimate of vertical and horizontal permeability. The above equation was derived based on several assumptions: an infinite wellbore, constant drawdown rate and steady state flow. The steady state flow condition cannot be satisfied in a low permeability formation, or unless a long test time is used. A constant drawdown rate is not reachable in practice because the tool needs time for acceleration and deceleration. The storage effect also makes it difficult to reach a constant drawdown rate. The infinite wellbore assumption excludes the wellbore effect on the non-spherical flow pattern, making their method not inapplicable to high kH/kV cases. The cases of kH/kV<1 were not presented in Dussan. The method works only in a homogeneous formation. However, their method does not have any procedure to check if the condition of homogeneous formation can be satisfied for a real probe test. The present invention addresses all of these limitations.
U.S. Pat. No. 5,265,015 to Auzerais et al. teaches determination of vertical and horizontal permeabilities using a special type of probe with an elongate cross-section, such as elliptic or rectangular. Measurements are made with two orientations of the probe, one with the axis of elongation parallel vertical, and one with the axis of elongation horizontal. The method requires a special tool configuration. To the best of our knowledge, there does not exist such a tool and it is probably difficult or expensive to build one. The present invention does not require a special tool, and such tool is available, for example, the one described in U.S. Pat. No. 6,478,096 to Jones et al.
U.S. Pat. No. 5,703,286 to Proett et al. teaches the determination of formation permeability by matching the pressure drawdown and buildup test data (possibly over many cycles). There is a suggestion that the method could be modified to deal with anisotropy and explicit equations are given for the use of multiple probes. However, there is no teaching on how to determine formation anisotropy from measurements made with a single probe. Based on the one equation given by Proett, it would be impossible to determine two parameters with measurements from a single probe. It would be desirable to have a method of determination of anisotropic permeabilities using a single probe. The present invention satisfies this need.